1. Field of the Invention
The present invention relates to a specific sectioned tube comprising an outer tube that is an outer body in a cylindrical round form having the same cross-section over an entire length, and at least two inner tubes inside the outer tube such that an inner space enclosed by the outer tube is sectioned into four or more subspaces (hereinafter, simply referred to as an inner-space-sectioned tube), and more specifically, to an inner-space-sectioned tube which has excellent sealing characteristics at sealing surfaces, plane contact surfaces between the outer and inner tubes, including between inner tubes themselves, while allowing manufacturing costs to be reduced by decreasing the number of cold drawing passes and enabling to efficiently increase tube strength despite a light weight structure, and a method for manufacturing the same.
2. Description of the Related Art
In recent years, among steel tubes used as mechanical tubes, a tubular product that has a specific function has been developed, responding to a growing demand for development of automobile parts friendly to global environment. For example, for a tube to be used for valve locker shafts in automobile engines, a function to transport working fluids for opening/closing the valve is required, while there is a need to delicately control the opening/closing of the valve from the view point of enhancing fuel efficiency and energy conservation, and thus it becomes necessary to provide the inner space of the tube with a plurality of fluid channels that are isolated from each other.
Incidentally, from the necessity to secure further controllability, it has been required to construct a plurality of independent fluid channels by dividing the inner space of the tube into at least four (4) subspaces, substituting the conventional structure where two fluid channels in a longitudinal orientation are independently provided within the inner space of the tube. All the more, although this is a development work of parts having specific functions, it is also strongly demanded to reduce the weight of the automobile part and to cut costs.
Conventionally, when a tube having a plurality of independent fluid channels inside itself is manufactured, there has been applied a method such that a solid round bar is subjected to gun-drilling etc. to have a plurality of boreholes drilled in a longitudinal orientation, and these boreholes are used as fluid channels. According to this method, the number of boreholes is not limited to two or four, so any suitable number of boreholes can be made upon the development requirement.
However, when the solid round bar is drilled, there is a limit in depth of the borehole that can be drilled, and it is extremely difficult to form long fluid channels, while machining accuracy is hardly assured because of difficulty in drilling in a longitudinal orientation parallel to the longitudinal axis. Moreover, the tube thus made should occasionally have a redundant wall thickness, which prevents its weight from being lightened, thus becoming of an issue.
In order to solve these problems, Japanese Patent Application Publication No. 7-144221 proposes tubes having various cross-sectional configurations wherein, by providing inside tubes such that some segments of their outer peripheral surfaces are pressure-bonded onto some segments of an inner peripheral surface of an outside tube, an inner space confined by the outside tube is partitioned by the tube-wall segments of the inside tubes that are not pressure-bonded onto the outside tube.
FIG. 1 is a cross-sectional view illustrating an example of a cross-sectional configuration of a tube having four independent fluid channels within its inner space. The cross-sectional configuration illustrated in FIG. 1 exhibits a structure that two lengths of round inside tubes 2 are provided inside an outside tube 1, such that the inside tubes 2 are partly brought into contact with each other, and also are configured so as to partly bring the outer peripheral surfaces thereof to contact with the inner peripheral surface of the outside tube 1. This tube structure is regarded as allowing a multiple fluid channels to be formed by inserting two or more lengths of inside tubes 2 into the outside tube 1, without having to limit the number of inside tubes 2.
FIG. 2 is a cross-sectional view illustrating an alternative example of the cross-sectional configuration of a tube having four independent fluid channels within its inner space, which is proposed in the above Patent Application Publication. The cross-sectional configuration in FIG. 2 exhibits a structure that: each inside tube 2 is designed to have a cross-section of the same fan shape, while an outside tube 1 is a cylindrical round tube: in providing four fluid channels, each of four lengths of inside tubes 2 having the fan shape with the center angle of 90 degree is pressure-bonded with two others to thereby construct an internal partition wall of cross-like shape in cross-section.
And, it is perceived that such a tube structure illustrated in FIG. 2 can render an incurred stress distribution even at each portion of the tube and alleviate the extent of local stress concentration since each inside tube has a fan shape, exactly the same with each other, thus enabling to effectively impart reinforcement in two directions at right angles to each other even in such a simple structure.
When a tube having four or more independent fluid channels therein is manufactured, the tube structure illustrated in the above FIG. 1 can be adopted so that a drawing process is applied while, for example, two lengths of smaller-size inside tubes are inserted into the outside tube, thus enabling to make the tube having four channels.
However, when the tube is used as the valve locker shaft for use in an automobile engine, it is mandatory that lubricating oils, working fluids for opening/closing the valve and the like inside the tube must not be mixed with each other and must be transported separately. In this regard, the tube structure shown in the above FIG. 1 is not sufficient in sealing characteristics at the contact positions both between the outside and inside tubes and between the inside tubes themselves, thus being unable to exhibit sufficient sealing performance demanded for the valve locker shaft.
Likewise, in manufacturing the tube having four independent fluid channels therein, the tube structure shown in the above FIG. 2 can render an incurred stress distribution even at each portion of the tube, while allowing the inner partition wall to be formed in a manner of stacking the walls of the inner tubes adjacent with each other to thereby enhance strength of the inner partition wall. Nonetheless, the tube structure shown in the above FIG. 2 raises the issue that it not only causes manufacturing costs to rise, but also causes the available channel space inside the tube to be narrowed, while preventing the weight reduction.
Now, with regard to manufacturing costs, the tube illustrated in the above FIG. 2 is to be made such that four lengths of inside tubes 2 are inserted and set into the outside tube 1, followed by a cold drawing process as being set, wherein, in common practice, one (1) pass of cold drawing is applied for making the outside tube 1 itself, and two (2) passes of cold drawing are needed for making each inside tube 2. Accordingly, when the tube shown in the above FIG. 2 is made by the cold drawing process, the total number of cold drawing passes amounts to ten (10), thus resulting in raising manufacturing costs enormously.
Next, as regards securing the space inside the tube, by way of example, the tube to be used as the valve locker shaft for use in the automobile engine is small in size such that the outside diameter of the outside tube is designed to be about 20 mm. And, in order to secure close fit between the outside tube and inside tubes and to secure strength of the inside tube wall, the wall thickness thereof needs to be in the range of 1.0 to 1.2 mm. In such a dimensional arrangement, forming the inner partitioned wall in a manner of stacking the walls of inside tubes adjacent with each other makes the available space for four fluid channels to be narrow, thus making it difficult to ensure adequate controllability.
Furthermore, regarding weight reduction of parts, despite that the outside tube of small size is utilized, the reduction of weight becomes difficult because of the necessity to form the inner partitioned wall in a manner of stacking the walls of the inside tubes adjacent with each other.